Method for obtaining flat and stress-free magnetic strip



Dec. 15, 1964 c. E. WARD ETAL 3,161,225

METHOD FOR OBTAINING FLAT AND STRESS-FREE MAGNETIC STRIP Filed Dec. 29,1961 2 Sheets-Sheet 1 Fig. 1

4 I F1g.2 5

I0 4 5 CHESTER E. WA IZD EL Z O BY were/2 W CARPENTER,

Fig. 3 ATTORNEYS.

Dec. 15, 1964 c, E. WARD ETAL 3,161,225

METHOD FOR OBTAINING FLAT AND STRESS-FREE MAGNETIC STRIP Filed Dec. 29.1961 2 Sheets-Sheet 2 15 75 /5q /4 760 l6 l6 Fig. 5

INVENTOR.

CHESTER E. WARD AND V/croe W CARPENTER,

ATTORNEYS,

United States Patent Ofifice 3,161,225 Patented Dec. 15, 1964 3,161,225METHOD FOR GBTAKNENG FLAT AND STRESS-FREE MAGNETEQ STRIP (Ihester E.Ward and Victor W. Uarpenter, Middletown,

@hio, assimors to Arrnco Steel (Iorporation, ,Middletown, Qhio, acorporation of @hio Filed Dec. 2h, i961, Ser. No. 163,283 tjlairns. (Cl.153-86) This invention has to do with the production of magnetic stripmaterials which require extreme flatness and freedom from stress, andrelates more specifically to the removal of coil set and theminimization of stresses,

both mechanical and thermal, encountered in the production of magneticstrip materials.

In the thermal flattening of magnetic strip material, two distinctproblems are encountered The first is strip shape. Invariably, the coilsto be processed exhibit a lack of the flatness required in laminationsfor transformers and other applications of magnetic material. Whilevarious methods have hitherto been proposed to flatten the strip, suchas hot roller leveling and the use of catenary units, these methods relyheavily upon tension and the resultant elongation to flatten thematerial and are designed primarily to remove such irregularities aslongitudinal troughs, buckles, and the like commonly encountered inrolled and heat treated metallic strip. Such methods are effective inflattening strip but the operation often is so severe that all of thedeleterious effects of mechanical stresses are not removed and thematerial is then unsatisfactory for magnetic applications.

The second important problem encountered in the production of magneticstrip material is stress. The stresses involved are mechanical stresses,such as those occasioned by the physical processing of the strip, andthermal stress resulting from fast and non-uniform cooling of the stripafter the heating step in the processing. These two types of stressescan be complementary and under extreme circumstances result in permanentimpairment of magnetic properties, i.e., original properties are notrecoverable with heat treatment for stress-relief. Even if not permanently damaging, residual stresses from processing may be undulydifiicult to remove with stress-relief annealing and will, if notcompletely removed, cause decreased etticiency in the magneticapplication. Consequently, it is highly desirable that prior processingstresses, inclusive of those introduced by flattening of the strip, bekept to an absolute minimum.

Accordingly, a principal object of the instant invention is theprovision of procedures by means of which both coil set and the stressesintroduced during processing of the magnetic strip material may beeffectively eliminated, thereby providing strip having extreme flatnesswithout any appreciable loss in magnetic properties.

A further object of the instant invention is the provision of a simpleand inexpensive procedure by means of which coil set or longitudinalcurvature can be effectively removed from magnetic strip materialwithout inducing mechanical stresses which cannot be relieved in thesame operation. I

Still a further object of the invention is the provision of a techniquefor obtaining extreme flatness in magnetic strip material while at thesame time relieving both mechanical and thermal stresses introduced inthe strip during prior processing or as an incident to the removal ofcoil set, such techniques embodying the passage of the strip materialthrough a heat treating furnace wherein a controlled reverse curvatureis introduced into the strip to offset coil set, such reverse curvaturebeing introduced while the strip is being subjected to a stressrelieving heat treatment and advanced through the furnace with a minimumof stress-producing tension and flexing of the strip.

The foregoing together with other objects of the instant invention whichwill appear hereinafter or which will be apparent to the skilled workerin the art upon reading this specification, are accomplished by thoseprocedures and method steps which will be described more fullyhereinafter.

Reference is now made to the accompanying drawings wherein:

FIGURE 1 is a schematic representation of one form of apparatus suitablefor carrying out the procedures of the instant invention.

FIGURE 2 is a schematic diagram similar to FIGURE 1 illustrating analternative form of apparatus.

FIGURE 3 is a schematic diagram of another form of apparatus.

FIGURE 4 is a schematic diagram of still another form of apparatushaving a two part pass line.

FIGURE 5 is a schematic diagram of yet another form of apparatus.

The flattening procedures of the instant invention are based upon thetheory that if strip containing coil set (longitudinal curvature) isflattened, transverse curvature develops approximately in the amount R=R /a, where R =transverse radius of curvature, R =longitudinal radiusof curvature and U=POlSSOn S ratio. Poissons ratio is deimcd as theratio of the transverse contraction of a strained test specimen to itslongitudinal elongation. Since a metal strip does not remain fiat orplanar in cross-section when thes trip is straightened, but rather formsa trough with the edges high, it has been found that a very slightreverse bend will restore the planar condition of the cross-section.Such reverse bend can be readily induced by causing the strip to passover a roll positioned to deflect the strip from its path of travel in asingle direction only in opposition to the coil set of the strip. To beeffective, such reverse bending must be carried out at a temperaturewhere the strip material deforms plastically with case. It has beenfound that where this is done an essentially flat surface will bedeveloped without material damage to the magnetic properties of thestrip. This statement should be qualified by pointing out that theplastic deformation which can be tolerated appears to apply only todeformation incurred by the single direction bending operation of theinstant invention. That is, the stresses induced by the single reversebend at the annealing temperature will be self-healing, Whereas it hasbeen found that even very small amounts of plastic strain of the orderof 0.05% elongation caused by multiple bending or by tension will resultin irrecoverable damage to the magnetic properties of the strip.

Referring now to FIGURE 1 of the drawings, a thermal-flattening furnaceis indicated at 1, such furnace being of conventional length which,while variable, may be on the order of feet. The leading end of thefurnace, i.e., approximately the first 60 feet, is for heating andsoaking; whereas the remainder of the furnace is for cooling. The strip2 is conducted through the furnace on a series of spaced apart hearthrolls 3 which are preferably driven, the strip being fed into thefurnace from a pay oil reel 4 and removed by means of rewind reel 5. Itwill be understood, of course, that the magnetic strip material willoften be coated with a thin layer of a composition capable of reactingwith the surface material of the strip while at an elevated temperatureto produce an adherent solid insulating film of high resistivity on thesurfaces of the strip material. For example, box annealed coils ofsilicon steel strip may be first uncoiled and passed through a scrubberand then into a coating tank where a coating composition, such as anaqueous phosphoric acid solution, is applied and.

inch, preferably approaching 100 lbs. per square inch. The reduction intension is greatly facilitated by driving the hearth rolls.. Desirably,the tensile stress exerted on the strip should be insufiicient incombination with thermal stress to case creep or plastic elongation ofthe strip. Stresses of a magnitude to cause creep or plastic trated inFIGURE 1, the reverse bending of the strip can be induced -by elevatinga single roll 6 above the pass line of the hearth rolls 3. Preferably,the roll 6-which can.

be referred to as a flattening rollalso will be driven and may comprisea 4 inch 'diarneter'cylindrical roll.

elongation have been found unnecessary to eflect flattening inaccordance with the instant invention, and in ad-v dition they causeirreparable damage to the magnetic properties of the strip.

Turning now to FIGURE 2 of the drawings, there is illustrated amodification of the invention wherein the single reverse bending of thestrip is induced by means of a series of adjacent flattening rolls 7, '8and 9 displaced upwardly from the pass line of the strip, as: defined bythe hearth rolls 3. As before, the purpose of this arrangement is tointroduce a single bend in the strip in In an exemplary embodiment ofthe invention, the flattening roll'was positioned approximately feetfrom the exit end of the heating and soaking zone of the furnace. Thereverse bending which takes place is unidirectional, that is, the stripis deflected from the pass line in a single direction only, and whilethe strip returns to the pass line 'upon passage over the flatteningroll, it is not bent in more than one direction nor more than one time.

This is important since multiple bending of the strip, whether it be inthe same direction or in opposite directions, as where the strip wouldbe fed over and under adjacent rolls, acts to introduce undesirablemechanical stresses which can permanently impair the magnetic propertiesof, thestrip.

The amount of flattening rollelevation above the passline of the stripwill vary depending upon the diameter of the flattening roll; however,where a 4 inch diameter contacts the strip and hence the greater thetimethe transverse cross-section is planar. It. will also be evidentthat, fora given roll elevation, the smaller the roll. diameter thesharper the bend in the strip. Consequently, the amount of penetrationwill vary depending upon numerous factors; and the combinations of rollsize and roll penetration that will produce. satisfactory-results arenumerous. a r

It 'is also contemplated, that the: flattening roll will'be movablymounted so that it may be automatically. moved 7 relative to the passline of the 'strip'in accordance with" the diameter; of the coil beingunwound That is, as

1 to proportionally increase the degree of reverselbending i the strip,las defined by the sets of-rolls 15 and 16, but the strip is unwoundfrom the coil the diameter of the 3 coil will decrease with eachsucceeding convolution, and] the coil set and resultant transversecurvature of the strip 5 will increase accordingly Consequently, it isdesirable opposition to its coil set. In effect, the cluster offlattening rolls 7, 8 and 9 define a single large diameter bendingsurface which enables the user to more closely calibrate the correctivedeformation of the strip.

. It should be evident that, instead of elevating one or more ofthe'flattening rolls, the same effect may be brought about by depressingone or more rolls below the pass line of the strip, in which event thestrip will be payed off and recoiled from the top of the coil instead offrom the bottom of the coil.

It should be understood that the term singlereverse bend as used in thedescription and claims does not preclude some flexing of the strip inthe direction of original 'stripcurvature or coil 'set as an incident ofthe bending operation. In the embodiments of ,the inventionillustratedin FIGURES 1 and2, some flexing ofthe strip in the direction of coil settakes place as the strip moves upwardly about the flattening roll andthen returns to the hearth rolls 3. While such flexing of the strip hasnot been found to adversely affect the'eifectiveness of the flatteningrolls in removing coil set, other furnace arrangements may be. employedin which flexing of the strip in the direction of coil set is eliminatedentirely or materially reduced. a

Foriexample, as, seen in FIGURE 3, the coils 4 and 5 may be placedbeneath the furnace and the strip lead over a series" of hearth rolls 10arranged to define an inverted U-shaped' path of travel. With thisarrangement, there .is no flexing of the strip in the direction of coilset as itpasses through the furnace. Alternatively, the coils may beplaced to the sides or above the furnace and the U-shaped rolls orientedaccordingly.

In the embodiment of the invention illustrated in FIG- URE 4, the'str'ipis taken over a series of hearth rolls 11 .to an elevated flatteningroll 12, but in this'instance the pass line of the strip beyond theelevated flattening roll is raised .to the level of the flattening roll12, the strip 7 being conveyed from the furnace on raised hearth rolls13.

A further form of the invention is illustrated in FIG URE 5 wherein thehearth roll 14 lies in the pass line of gle reverse bendingof the stripwithout elevating or dcto compensate for the progressivelyiincreasingcoil set.

This may be readily accomplished by a screw jack mounting5of the.flatteningroll; with the screw driven in timed;

and calibrated relation to th pam t s 1 being unwound and its feeding"speed.-

a r Y strip to advance it through'ihe' furnace. For-"optimum results,the tensionlshouldbe, essentially norg r'eater thanv that required toadvancethe strip. Here again, it'is impossible to state precisetensions, although ideally the pressingany of the rolls. The 'strip willpass over or under the isolated roll 14 depending upon whether the stripis payed off and reeled in from the bottom or top of the coil,respectively, It may be noted that while 'asinglefree 'catenary withinthe hot zone of :a furnace has been used for manytyears, it has not"been'pre'viously known to position a hearth roll in'such a mannerjthatone or two shorter catenaries-,may be formed and' con trolledtoefiectfthe desired reverse bending of the-strip.

As should now be"apparent, the instant invention contemplatesproceduresbyfmeans of which extreme flatness ,eanb'e produced inmagnetic strip material, such asv grain tensile stress jshould bewellsbelow r1000, lbs;.fp'er square aori ented silicon steel, by bendingthe "strip in one direction only-in opposition to coil setwhile atstress relieving temperature and in the absence of excessive tensilestresses. The flatness characteristics of the strip are greatly enhancedwhile at the same time the type of magnetic damage which results frommechanically induced stresses is essentially eliminated. It will beunderstood that deformations of the strip, such as wavy edges, sagbuckles, and the like, will be minimized by processing controls ahead ofthe thermal flattening treatment. Thermal stresses, such as thoseinduced by fast and nonuniform cooling of the strip, will be controlledin the cooling section of the furnace. If desired, a cooling hood may beprovided in order to provide more gradual cooling of the strip.

The instant procedures are particularly applicable to the treatment ofcoated strip since the strip is deformed only in one direction and iscontacted only on one side. Consequently, the scratching, cracking, andflaking of the insulating'films during the processing and handling ofthe strip is minimized, thereby providing on the magnetic stripmaterials more uniform insulating films having high dielectricproperties.

Having thus described the invention in certain exemplary embodiments andwith the understanding that modification may be made therein withoutdeparting from its spirit and purpose, what it is desired to secure andprotect by Letters Patent is:

1. A process for flattening magnetic metallic strip material wherein thestrip, as an incident of its processing, has been coiled and boxannealed, said process comprising unwinding .the coil strip and passingit through a heat treating furnace and subjecting it to a stressrelieving heat treatment and, as said strip passes through said furnace,introducing a single reverse bend in said strip in opposition to thecoil set thereof by contacting one side only of the strip and deflectingit in the direction opposite to the coil set, said strip being advancedthrough the furnace at a tension insufiicient to produce creep orplastic elongation of the said strip which would impair its magneticproperties.

2. The process claimed in claim 1 wherein the pass line of said strip isdefined by a series of spaced apart hearth rolls transversely disposedwith respect to the path of travel of the strip and upon which the stripis supported and advanced through the furnace, and wherein the singlereverse bend in said strip is effected by means of at least oneflattening roll positioned to penetrate the pass line of said strip.

3. The process claimed in claim 2 wherein the penetration of saidflattening roll is progressively increased in proportion to the decreasein the diameter of the coil being treated as the strip material makingup said coil is payed out.

4. The process claimed in claim 3 wherein said flattening roll has adiameter on the order of 4 inches and is mounted to selectivelypenetrate the pass line of said strip by a distance of from about 2inches to 6 inches.

5. A process for the thermal flattening of grain oriented silicon steelto remove coil set and at the same time minimize mechanical stressesintroduced therein by such processing, said process comprising the stepsof unwinding the coil strip and passing it through a heat treatingfurnace and, while said strip is passing through said furnace,subjecting it to a single reverse bend in opposition to the coil set ofsaid strip by contacting one side only of the strip and deflecting it inthe direction opposite to the coil set, said strip being advancedthrough said furnace at a tension insufficient to produce creep orplastic elongation of the strip which would impair its magneticproperties.

6. The process claimed in claim 5 wherein said stress relieving heattreatment is at a temperature effective to provide plastic bending ofthe strip with case.

7. The process claimed in claim 6 wherein the strip enters the furnacealong a first pass line, wherein the single reverse bend is introducedby means of at least one flattening roll displaced vertically from saidfirst pass line, and wherein said strip, upon passage beyond saidflattening roll, is conveyed along a second pass line in essentiallyhorizontal alignment with said flattening roll.

8. The process claimed in claim 6 wherein said strip is uncoiled frombeneath the furnace and is passed upwardly through the furnace in aninverted U-shaped path of travel.

9. The process claimed in claim 6 wherein said strip is passed throughthe furnace while supported on a series of spaced apart hearth rollsdefining an essentially horizontally disposed pass line, wherein atleast one of said hearth rolls is separated from the adjoining hearthrolls by a distance sutficient to define a catenary therebetween.

References Cited in the file of this patent UNITED STATES PATENTS2,868,702 Brennan Jan. 13, 1959 2,922,460 Schwendenwein Jan. 26, 19602,980,561 Ford et al Apr. 18, 1961

1. A PROCESS FOR FLATTENING MAGNETIC METALLIC STRIP MATERIAL WHEREIN THESTRIP, AS AN INCIDENT OF ITS PROCESSING, HAS BEEN COILED AND BOXANNEALED, SAID PROCESS COMPRISING UNWINDING THE COIL STRIP AND PASSINGIT THROUGH A HEAT TREATING FURNACE AND SUBJECTING IT TO A STRESSRELIEVING HEAT TREATMENT AND, AS SAID STRIP PASSES THROUGH SAID FURNACE,INTRODUCING A SINGLE REVERSE BEND IN SAID STRIP IN OPPOSITION TO THECOIL SET THEREOF BY CONTACTING ONE SIDE ONLY OF THE STRIP AND DEFLECTINGIT IN THE DIRECTION OPPOSITE TO THE COIL SET, SAID STRIP BEING ADVANCEDTHROUGH THE FURNACE AT A TENSION INSUFFICIENT TO PRODUCE CREEP ORPLASTIC ELONGATION OF THE SAID STRIP WHICH WOULD IMPAIR ITS MAGNETICPROPERTIES.